1. Field of the Invention
The present invention relates to an optical contour measuring apparatus for optically measuring the contour of an object.
2. Description of the Related Art
FIG. 5 shows the optical system of a contour measuring apparatus utilizing optical heterodyne interference. A Zeeman laser 100 serving as a light source radiates two superposed orthogonally polarized light beams of slightly different frequencies. The radiated light beams are divided by a nonpolarizing beam splitter 101. Portions of the light beams travel through a polarizer 102 and fall on an optical sensor 103. The optical sensor 103 provides a beat frequency signal representing the frequency of a beat generated by the combination of the two light beams of slightly different frequencies. The other portions of the light beams are divided into polarized light beams having a plane of oscillation parallel to the polarizing sheet (P-polarized beams) and polarized light beams having a plane of oscillation perpendicular to the sheet (S-polarized beams). The S-polarized beams traveled through a 1/4-waveplate 105 are reflected by a mirror 106, and the reflected S-polarized beams fall again on a polarizing beam splitter 104. The P-polarized beams traveled through a 1/4-waveplate 107 are focused by an objective lens 108 on the surface of an object 109. Then, the P-polarized beams reflected by the surface of the object 109 travel through the objective lens 108 and the 1/4-waveplate 107 and fall on the polarizing beam splitter 104. The reflected S and P-polarized beams traveled through a polarizer 110 fall on an optical sensor 111. Then, the optical sensor 111 provides a beat frequency signal representing the frequency of a beat generated by the combination of the light beams of slightly different frequencies. The beat frequency signal provided by the optical sensor 103 and that provided by the optical sensor 111 have different phases, respectively. Since the phase difference between the respective phases of the beat frequency signals varies according to the contour of the object 109, the contour of the object 109 can be measured through the measurement of the variation of the phase difference.
This conventional contour measuring apparatus is able to measure the contour of the object 109 at a very high accuracy provided that the surface of the object 109 is smooth, but is unable to measure the contour at a satisfactory accuracy when the surface of the object 109 is rough because the light beam is reflected irregularly by the rough surface of the object 109 and hence the wavefront of the light beam is disturbed, so that accurate detection of the beat frequency is impossible.
A trigonometric contour measuring apparatus has been proposed for the measurement of the contour of an object having a rough surface, but the measuring accuracy of the trigonometric contour measuring apparatus is unsatisfactory, and the dimensions of the apparatus are increased inevitably when the resolution of the apparatus is enhanced for accurate measurement.